The structure of multi-walled carbon nanotubes was originally reported by Iijima, (Iijima, S. “Helical Microtubules of Graphitic Carbon.” Nature 354, 56-58 (1991)) to be comprised of concentric single-walled tubes, known as the Russian doll model. Most growth mechanisms involve the dissolution of carbon and subsequent growth from transition metal catalyst particles at high temperatures. (Amelinckx, S., et al. “A Formation Mechanism for Catalytically Grown Helix-shaped Graphite Nanotubes.” Science 265, 635-639 (1994)). The size of the catalyst dictates the diameter of the carbon nanotubes produced. An alternative theory suggests that multi-walled nanotubes could form via a scrolling mechanism. (Amelinckx, S., Bernaerts, D., Zhang, X. B., Van Tendeloo, G., Van Landuyt, J. “A Structure Model and Growth Mechanism for Multishell Carbon Nanotubes.” Science 267, 1334-1338 (1995), Zhou, O., et al. “Defects in Carbon Nanostructures.” Science 263, 1744-1747 (1994)).
Since transmission electron microscopy is a two-dimensional projection of a three-dimensional object, one cannot generally distinguish between nanoscrolls and nanotubes by direct observation. In fact, a scrolling mechanism would explain the odd number of carbon layers often seen when counting parallel carbon layers at 3.4 Å spacings in transmission electron micrographs. (Amelinckx, S., et al, ibid Science 267, 1334-338 (1995)). The concept that scrolling could potentially lead to nanotube-like structures gave us the inspiration to extend our recent work on making colloidal suspensions of layered compounds (Ding, Z., Viculis, L., Nakawatase, J., Kaner, R. B. “Intercalation and Solution Processing of Bismuth Telluride and Bismuth Selenide. Adv. Mater. 13, 797-800 (2001)) to Graphite.
The discovery of fullerenes (Kroto, H. W., Heath, J. R., O'Brien, S. C., Curl, R. F., Smalley, R. E. “C-60:Buckminsterfullerene”, Nature, 318, pp 162-163 (1985)) and carbon nanotubes (Iijima, S., “Helical Microtubules Of Graphitic Carbon”, Nature, 354 pp 56-58 (1991)) has sparked extensive research efforts into applications of nanomaterials (Yakobson, B. I., Smalley, R. E., “Fullerene Nanotubes: C-1000000 and Beyond”, American Scientist, 85, pp 324-337 (1997); Subramoney, S., “Novel Nanocarbons—Structure, Properties, and Potential_Applications”, Adv. Mater., 10, pp 1157-1171 (1998)). Proposed uses include composite reinforcements (Schaffer, M. S. P., Windle, A. H., “Fabrication and Characterization of Carbon Nanotube/poly (vinyl alcohol) Composites”, Adv. Mater., 11, pp 937-941 (1999)), hydrogen storage (Ye, Y., Ahn, C. C., Witham, C., Fultz, B., Liu, J., Rinzler, A. G., Colbert, D., Smith, K. A., Smalley, R. E., “Hydrogen Absorption And Cohesive Energy Of Single-Walled Carbon Nanotubes”, App. Phys. Lett., 74, pp 307-2309 (1999); Liu, C., Fan, Y. Y., Liu, M., Cong, H. T., Cheng, H. M., Dresselhaus, M. S., “Hydrogen Storage In Single-Walled Carbon Nanotubes At Room Temperature”, Science, 286, pp 1127-1129 (1999); Kong, J., Chapline, M. G., Dai, H., “Functionalized Carbon Nanotubes For Molecular Hydrogen Sensors”, Adv. Mater. 13, 1384-1386 (2001)), supercapacitors (Aldissi, M.; Schmitz, B.; Lazaro, E.; Bhamidipati, M.; Dixon, B., “Conducting Polymers In Ultracapacitor Applications”, 56th Annu. Tech. Conf.—Soc. Plast. Eng., (Vol. 2), pp 1197-1201 (1998); An, K. H.; Kim, W. S.; Park, Y. S.; Moon, J.-M.; Bae, D. J.; Lim, S. C.; Lee, Y. S.; Lee, Y. H. “Electrochemical Properties Of High-Power Supercapacitors Using Single-Walled Carbon Nanotube Electrodes”, Adv. Funct. Mater. 11, pp 387-392 (2001)), catalysis (Yu, R., Chen, L., Liu, Q., Lin, J., Tan, K.-L., Ng, S. C., Chan, H. S. O., Xu, G.-Q.,Hor, T. S. A. “Platinum Deposition On Carbon Nanotubes Via Chemical Modification”, Chem. Mater. 10, pp 718-722 (1998); (-Planeix, J. M.; Coustel, N.; Coq, B.; Brotons, V.; Kumbhar, P. S.; Dutartre, R.; Geneste, P.; Bernier, P.; Ajayan, P. M., “Application Of Carbon Nanotubes As Supports_in Heterogeneous Catalysis”, J. Am. Chem. Soc. 116, pp 7935-7936 (1994)) and nano-scale electronic devices (Tans, S. J., Verschueren, A. R. M., Dekker, C., “Room-Temperature Transistor Based On A Single Carbon Nanotube”, Nature 393, pp 49-52 (1998); Bachtold, A.; Hadley, P.; Nakanishi, T.; Dekker, C., “Logic Circuits With Carbon Nanotube Transistors”. Science 294 pp 1317-1320 (2001)). Most of these applications depend upon a reliable source of high-quality inexpensive nanomaterials. Since carbon nanotubes are currently synthesized using high temperature arc-discharge (Ebbeson, T. W., Ajayan, P. M. “Large Scale Synthesis Of Carbon Nanotubes”, Nature 358, pp 220-222 (1992)) or laser vaporization methods (Thess, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee, Y. H.; Kim, S. G.; Rinzler, A. G.; Colbert, D. T.; Scuseria, G. E.; Tomanek, D.; Fischer, J. E.;_Smalley, R. E., “Crystalline Ropes Of Metallic Carbon Nanotubes”. Science 273, pp 483-487 (1996)), their price is prohibitively high (on the order of $90 per gram or more) (Oger, G., “French Firm Hopes To Get PR Bounce Out Of Nanotubes In Tennis Rackets”, Small Times, Nov. 7, 2001,
http://www.smalltimes.com/document_display.cfm?document_id=2506)) which limits their use to small-scale applications, such as scanning tunneling microscopy tips.
A significant application is in the fabrication of hydrogen storage devices. The on-going depletion of our natural resources, especially fossil fuels, and worldwide environmental issues has sparked public concern over finding a clean and renewable energy source. Possible energy sources include nuclear, solar, hydro- and wind-power. The problem with all of these is that they cannot be used directly as a fuel. Hydrogen is an ideal candidate as both a fuel source and energy carrier because it is environmentally benign, highly efficient, convenient and versatile (Veziroglu, T. N. “Hydrogen Energy System As A Permanent Solution To Global Energy-Environmental Problems”, Chem. Ind. 53, pp 383-393 (1999); Veziroglu, T. N., Barbir, F. “Hydrogen: The Wonder Fuel”, Int. J. Hydrogen Energy 17, pp 391-401 (1992)). Eventually, hydrogen is expected to replace fossil fuels as the primary fuel for vehicles. Some of the major automobile manufacturers have committed to offering fuel cell automobiles sometime in the next few years (http://www.daimlerchrysler.com). Hydrogen can be used either in fuel cells or directly in internal combustion engines with only minor modifications.
The primary barrier for using hydrogen in vehicles is the lack of an efficient and safe storage technology. Ideally, a practical hydrogen storage unit would be lightweight, inexpensive, compact, environmentally safe and easily renewable. The Department of Energy Hydrogen Plan has set a standard for the amount of reversible hydrogen adsorption needed (Deluchi, M. “Hydrogen Fuel-Cell Vehicles (Institute of Transportation Studies”, Univ. California, Davis, 1992)). The ratio of stored hydrogen weight to system weight is set at 6.5 wt % hydrogen with a volumetric density of 62 kg H2/m3. These numbers have been calculated based on a fuel cell automobile needing 3.1 kg of H2 for a 500 km range (Deluchi, M. “Hydrogen Fuel-Cell Vehicles (Institute of Transportation Studies”, Univ. California, Davis, 1992). Currently, no storage technology meets these requirements.
Recent reports of high levels of reversible adsorption of H2 in carbon nanotubes (Dillon, A. C., Jones, K. M., Bekkedahl, T. A., Kiang, C. H., Bethune, D. S., Heben, M. J., “Storage of Hydrogen in Single-Walled Carbon Nanotubes”. Nature, 386, pp 377-379 (1997); Ye, Y., Ahn, C. C., Witham, C., Fultz, B., Liu, J., Rinzler, A. G., Colbert, D., Smith, K. A., Smalley, R. E., “Hydrogen Adsorption and Cohesive Energy of Single-Walled Carbon Nanotubes”. App. Phys. Lett. 74, pp 2307-2309 (1999); Liu, C., Fan, Y. Y., Liu, M., Cong, H. T., Cheng, H. M., Dresselhaus, M. S. “Hydrogen Storage in Single-Walled Carbon Nanotubes at Room Temperature”. Science, 286, pp 1127-1129 (1999)), alkali-doped graphite and pure and alkali-doped nanofibers, (Chambers, A., Park, C., Baker, R. T. K., Rodriguez, N. M. “Hydrogen Storage in Graphite Nanofibers”. J. Phys. Chem. B, 102, pp 4253-4256 (1998); Chen, P., Wu, X., Lin, J., Tan, K. L. “High H-2 Uptake By Alkali-Doped Carbon Nanotubes Under Ambient Pressure And Moderate Temperatures”, Science, 285, pp 91-93 (1999)) have sparked great excitement. The field accelerated in 1997 with the first report of single-walled carbon nanotubes with high reversible hydrogen storage capacity (Dillon, A. C., Jones, K. M., Bekkedahl, T. A., Kiang, C. H., Bethune, D. S., Heben,_M. J., “Storage Of_Hydrogen In Single-Walled Carbon Nanotubes”, Nature, 386, pp 377-379 (1997)). A hydrogen adsorption of 5-10 wt % was extrapolated for single-walled carbon nanotubes at 133_K from as-prepared arc evaporated carbon soot containing only 0.1-0.2 wt % nanotubes. A later report by another group showed an H:C ratio of 1.0 (8 wt %) for crystalline ropes of single-walled carbon nanotubes at 80 K and pressures greater than 12 MPa (Ye, Y., Ahn, C. C., Witham, C., Fultz, B., Liu, J., Rinzler, A. G., Colbert, D., Smith, K. A., Smalley, R. E., “Hydrogen Adsorption And Cohesive Energy Of Single-Walled Carbon Nanotubes”, App. Phys. Lett., 74, pp 2307-2309 (1999)).
Treated graphite nanofibers in the form of tubules, platelets and herringbone structures have a reported hydrogen adsorption in excess of 11, 45 and 67 wt %, respectively, at room temperature and pressure of 12 MPa (as yet unconfirmed) (Chambers, A., Park, C., Baker, R. T. K., Rodriguez, N. M., “Hydrogen Storage In Graphite Nanofibers”, J. Phys. Chem. B. 102, pp 4253-4256 (1998)). Lithium-doped multi-walled nanotubes under ambient pressures have a hydrogen uptake of 20 wt % but are chemically unstable (Chen, P., Wu, X., Lin, J., Tan, K. L. “High H-2 Uptake By Alkali-Doped Carbon Nanotubes Under Ambient Pressure And Moderate Temperatures”. Science, 285, pp 91-93 (1999)). Potassium-doped multi-walled carbon nanotubes can adsorb 14 wt % hydrogen at elevated temperatures (473-673 K). However Liu, et al. studied hydrogen adsorption measurements on relatively pure samples of single-walled carbon nanotubes, (Liu, C., Fan, Y. Y., Liu, M., Cong, H. T., Cheng, H. M., Dresselhaus, M. S. “Hydrogen Storage In Single-Walled Carbon Nanotubes At Room Temperature”. Science, 286, pp 1127-1129 (1999)). They obtained a hydrogen storage capacity of only 4.2 wt % at room temperature and a pressure of 10 MPa.
It is clear from surface area studies on conventional porous carbon materials that a correlation exists between high surface area and high hydrogen adsorption (Ye, Y., Ahn, C. C., Witham, C., Fultz, B., Liu, J., Rinzler, A. G., Colbert, D., Smith, K. A., Smalley, R. E. “Hydrogen Adsorption And Cohesive Energy Of Single-Walled Carbon Nanotubes”. App. Phys. Lett., 74, 2307-2309 (1999); Agarwal, R. K., Noh, J. S., Schwarz, J. A., “Effect Of Surface Acidity Of Activated Carbon On Hydrogen Storage”, Carbon, 25, pp 219-226 (1987)). This is not strictly the case for carbon nanotubes and nanofibers where nanostructure and nanopore size may play a more important role in adsorption. Physical studies have concluded that hydrogen is stored in carbon nanotubes both inside the tubes and in interstitial sites between adjacent tubes (Dresselhaus, M. S., Williams, K. A., Eklund, P. C. “Hydrogen Adsorption In Carbon Materials”, MRS Bull., 24, pp 45-50 (1999)). This can lead to a higher storage density than planar graphite. Another factor related to adsorption is the nanopore size. The kinetic diameter of hydrogen is 2.89 Å, therefore the tube diameters and interplanar spacing of graphite sheets should allow for facile entry and exit of hydrogen molecules (Chambers, A., Park, C., Baker, R. T. K., Rodriguez, N. M., “Hydrogen Storage In Graphite Nanofibers”, J. Phys. Chem. B, 102, pp 4253-4256 (1998)). Density functional calculations have been performed in order to determine whether hydrogen storage in carbon nanotubes is by physisorption or chemisorption (Lee, S. M., Lee, Y. H., “Hydrogen Storage In Single-Walled Carbon Nanotubes”. Appl. Phys. Lett., 76, pp 2877-2879 (2000)). It is believed that hydrogen is adsorbed via a chemisorption pathway and it has been predicted that hydrogen storage in nanotubes could exceed 14 wt % or 160 kg H2/m3, numbers well above that needed for commercial applications. Clearly, a low-cost method for mass production of suitable nanocarbon materials needs to be perfected so that hydrogen can be realized as the fuel of choice.
Another major application for nanostructured materials is in the formation of nanoelectronic components. Electrochemical capacitors are becoming increasingly important for both military and commercial applications needing both high power density and high energy density. High energy density relative to traditional capacitors enables uses such as burst power sources for the signal generation of electronic components. Since supercapacitors also have higher power density than batteries, they can be used to run power plants efficiently by rapidly storing excess energy produced at night and rapidly releasing it during peak demand hours of the day. Supercapacitors can store large amounts of energy via charge separation in the double layer formed within the microstructure and nanopores of high surface area material. The charge separation is distributed throughout the electrode volume in the double layer formed at the interface between the two electrodes.
An electrochemical capacitor is constructed like a battery, however in a capacitor both electrodes are generally made from the same material. The current flowing from any electrode is accounted for by Faradaic reactions, surface charge transfer and/or charging of the double layer. In a battery, most charge is stored via Faradaic reactions, while in a capacitor essentially all the charge is stored in the double layer (Shukla, A. K.; Sampath, S.; Vijayamohanan, K., “Electrochemical Supercapacitors: Energy Storage Beyond Batteries”, Curr. Sci., 79, pp 1656-1661 (2000)). Ions from the electrolyte approach the electrodes but do not react with them. In some cases ions can be adsorbed onto surfaces or intercalated into the electrodes. This later phenomenon enables much more charge to be stored relative to simple double layer capacitance. Here an additional pseudo-capacitance results from charge transfer reactions (Conway, B. E. Electrochemical Supercapacitors, Kluwer Academic/Plenum, New York, (1999)). Since pseudo-capacitance is diffusion related, it will contribute to high energy density while double layer capacitance leads to high power delivery and long cycle life. The differences between and electronic capacitor, a supercapacitor, such as can be fabricated using carbon nanostructured materials, and a battery are listed in Table 1 The key requirements needed for outstanding performance is electrode conductivity and accessible surface area.
TABLE 1CAPACITOR AND BATTERY CHARACTERISTICSELECTRONICSUPERDEVICECAPACITORCAPACITORBATTERYDischarge10−6-10−3 s1-30 s0.3-3 hrTimeCharge time10−6-10−3 s1-30 s1-5 hrEnergy<0.11-1020-100Density(W h/kg)Power Density>10,0001,000-2,00050-200(W/kg)Coulombic10090-9570-85Efficiency Cycle LifeInfinite>100,000500-2,000
Nanostructured materials have important applications in structural composites. The ultimate goal of composites is to make very strong, ultra-lightweight materials that can replace steel (Dai, L.; Mau, A. W. H. “Controlled Synthesis Of Modification Of Carbon Nanotubes and C60: Carbon Nanostructures For Advanced Polymeric Composite Materials”, Adv. Mater., 13, pp 899-913 (2001)). The weakest link in a composite material is the polymer matrix itself. The idea behind forming nanocomposites is to strengthen the preexisting polymer matrix, while maintaining its lightweight properties. Impregnating the matrix with a nano-material will transfer the load from the matrix to the nanomaterial (Calvert, P. “A Recipe For Strength”. Nature, 399, pp 210-211 (1999)). Past technology has focused on using macroscopic carbon fibers that are spun into long rods, with the graphite crystallites arranged along the axis of the rod, and then imbedded in the polymer matrix (Lake, M. L.; Ting, J.-M. “Vapor Grown Carbon Fiber Composites”, Carbon Mater. Adv. Technol. pp 139-167 (1999)). The limitation of this technique is that as strength increases, ductility is sacrificed as a result of the increased brittleness. An analogy is steel rods imbedded in concrete to reinforce it. The concrete is made stronger, but then has little flexibility. Nanocomposites utilizing the nanomaterials described herein would add strength without sacrificing flexibility.